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An atomic line filter (ALF) is an advanced optical band-pass filter used in the physical sciences for filtering electromagnetic radiation with precision, accuracy, and minimal signal strength loss. Atomic line filters work via the absorption or resonance lines of atomic vapors and so may also be designated an atomic resonance filter (ARF). The three major types of atomic line filters are absorption-re-emission ALFs, Faraday filters and Voigt filters. Absorption-re-emission filters were the first type developed, and so are commonly called simply "atomic line filters"; the other two types are usually referred to specifically as "Faraday filters" or "Voigt filters". Atomic line filters use different mechanisms and designs for different applications, but the same basic strategy is always employed: by taking advantage of the narrow lines of absorption or resonance in a metallic vapor, a specific frequency of light bypasses a series of filters that block all other light. Atomic line filters can be considered the optical equivalent of lock-in amplifiers; they are used in scientific applications requiring the effective detection of a narrowband signal (almost always laser light) that would otherwise be obscured by broadband sources, such as daylight.〔 They are used regularly in ''Laser Imaging Detection and Ranging'' (LIDAR) and are being studied for their potential use in laser communication systems. Atomic line filters are superior to conventional dielectric optical filters such as interference filters and Lyot filters, but their greater complexity makes them practical only in background-limited detection, where a weak signal is detected while suppressing a strong background. Compared to etalons, another high-end optical filter, Faraday filters are significantly sturdier and may be six times cheaper at around US$15,000 per unit. == History == The predecessor of the atomic line filter was the infrared quantum counter, designed in the 1950s by Nicolaas Bloembergen. This was a quantum mechanical amplifier theorized by Joseph Weber to detect infrared radiation with very little noise. Zero spontaneous emission was already possible for x-ray and gamma ray amplifiers and Weber thought to bring this technology to the infrared spectrum. Bloembergen described such a device in detail and dubbed it the "infrared quantum counter". The media of these devices were crystals with transition metal ion impurities, absorbing low-energy light and re-emitting it in the visible range.〔 By the 1970s, atomic vapors were used in atomic vapor quantum counters for detection of infrared electromagnetic radiation, as they were found to be superior to the metallic salts and crystals that had been used. The principles hitherto employed in infrared amplification were put together into a passive sodium ALF. This design and those that immediately followed it were primitive and suffered from low quantum efficiency and slow response time. As this was the original design for ALFs, many references use only the designation "atomic line filter" to describe specifically the absorption-re-emission construction. In 1977, Gelbwachs, Klein and Wessel created the first active atomic line filter.〔 Faraday filters, developed sometime before 1978, were "a substantial improvement" over absorption-re-emission atomic line filters of the time.〔 The Voigt filter, patented by James H. Menders and Eric J. Korevaar on August 26, 1992, was more advanced. Voigt filters were more compact and "() be easily designed for use with a permanent magnet".〔 By 1996, Faraday filters were being used for LIDAR.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Atomic line filter」の詳細全文を読む スポンサード リンク
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